IEEE 802.16e system which deals with an international standard for broadband wireless access system consisted only of MSS (Mobile Subscriber Station) as mobile station (MS), base station (BS) and ASA (Authentication Service Authorization), in contrast to 2G or 3G mobile communication system having hierarchical structure such as HLR, VLR, MSC, BSC, RNC, and so on. And, the IEEE 802.16e system defines common physical (PHY) layers and medium access control (MAC) layers in the BS and MSS.
Hereinafter, for convenience to explain, let ‘MS’ collectively refer to any one of mobile station, mobile subscriber station, subscriber station, and so on.
FIG. 1 shows an example of frame structure of OFDMA physical layer in the broadband wireless access system.
As show in the FIG. 1, a downlink subframe starts with a preamble for use in synchronization and equalization in the physical layer. And, the downlink subframe comprises downlink map (DL-MAP) message and uplink map (UL-MAP) message defining the location and usage of downlink and uplink bursts. So, the downlink subframe defines whole structure of the frame through the DL-MAP and the UL-MAP.
Tables 1 and 2 show an example of DL-MAP and UL-MAP.
TABLE 1SyntaxSizeNotesDL-MAP_Message_Format( ) {Management Message Type = 28 bitsPHY Synchronization FieldvariableSee appropriate PHYspecification.DCD Count8 bitsBS ID48 bits Begin PHY Specific Section {See applicable PHYsection.for(i= 1; i <= n;i++) {For each DL-MAP element1 to n.DL-MAP_IE( )variableSee corresponding PHYspecification.}}if !(byte boundary) {Padding Nibble4 bitsPadding to reach byteboundary.}}
TABLE 2SyntaxSizeNotesUL-MAP_Message_Format( ) {Management Message Type = 38 bitsUplink Channel ID8 bitsUCD Count8 bitsAllocation Start Time32 bits Begin PHY Specific Section {See applicable PHYsection.for(i= 1; i <= n; i++) {For each UL-MAP element1 to n.UL-MAP_IE( )variableSee corresponding PHYspecification.}}if !(byte boundary) {Padding Nibble4 bitsPadding to reach byteboundary.}}
DL-MAP message, shown in the table 1, defines the usage of each burst for the downlink section in burst mode physical layer, and UL-MAP message, shown in the table 2, defines the usage of each burst for the uplink section.
Information elements (IEs) consisting the DL-MAP message includes DIUC (Downlink Interval Usage Code), CID (Connection ID) and location information of the burst (ex, subchannel offset, symbol offset, the number of subchannels and the number of symbols). And the location information of the burst makes user to identify a downlink traffic region.
And, IEs consisting the UL-MAP message include CID, UIUC (Uplink Interval Usage Code) and duration. The UIUC defines the usage of each the CID, and the duration defines the location of an allocated region. The usage of each region is defined by the UIUC used in the UL-MAP. And, each allocated region starts after the duration, defined in the UL-MAP IE, from the start position of previous region.
The above mentioned UIUC can be expressed as a following table 3.
TABLE 3UIUCUsage 0FAST-FEEDBACK Channel1-10Different burst profiles11Extended UIUC 2 IE12CDMA bandwidth request13PAPR reduction request14CDMA allocation IE15extended UIUC
Specifically, table 3 indicates OFDMA UIUC, and this expresses the usage of uplink data burst. For example, in the OFDMA system, UIUC 1-10 is a region for data burst, and UIUC 12 is used for CDMA resource allocation.
And, a following table 4 shows an extended UIUC which corresponds to UIUC 15 of the table 3.
TABLE 4ExtendedUIUCUsage00Power_control_IE01Mini-subchannel_allocation_IE02AAS_UL_IE03CQICH_Alloc_IE04UL_Zone_IE05PHYMOD_UL_IE06MIMO_UL_Basic_IE07UL_PUSC_Burst_Allocation_in_Other_Segment_IE08Fast_Ranging_IE09UL_Allocation Start IE0B . . . 0FReserved
At present, as shown in the table 4, 10 extended UIUCs are presented by subcodes.
Based on the above explanation, the method for handover between heterogeneous networks can be explained as follows.
IEEE 802.21 which deals with the media independent handover (MIH) between heterogeneous networks has the purpose of improving convenience of a user by providing seamless handover and service continuity between heterogeneous networks. And, the basic requirements of the IEEE 802.21 are MIH function, event service (ES), command service (CS) and information service (IS).
The MS for the above system is a multi-mode node which supports one or more interface type. And, the interface type can be one or more of the followings.                Wire-line type interface such as 802.3 based ethernet        IEEE 802.XX based Wireless interface                    802.11            802.15            802.16                        Interface defined by a cellular standardization organization such as 3GPP and 3GPP2        
FIG. 2 shows an example of the multi-mode MS capable of such handover between heterogeneous networks.
As shown in the FIG. 2, multi-mode MS comprises physical (PHY) layers and media access control (MAC) layers for the each mode. And, MIH function is a logical object, and can be located freely, because it can interface with each layer through service access point (SAP) in the protocol stack
Media independent handover (MIH) should be defined between 802 type interfaces, or between 802 type interface and non 802 type interface. And, a protocol for supporting mobility of the upper layer, such as mobile IP and session IP, should be supported for handover and seamless service.
On the other hand, IEEE 802.21 standard has another purpose of making various handover methods, which can be classified as “break before make” or “make before break”, and which can be performed efficiently. Media independent handover function (MIHF) provides an asymmetric service such as media independent event service (MIES) and a symmetric service such as media independent command service (MICS) through well defined service access point. Media independent handover technique is consisted of three MIHF services and MIH protocol. The three major MIHF services are MIES, MICS and media independent information service (MIIS).
Among the above, MIES deals with information which transported from link layer to upper layer, and the upper layer can receive this information by authentication process. Here, the upper layer comprising mobility management protocol may need to receive the link layer information such as information indicating the handover will be performed soon, or information indicating the handover was just finished, for predicting and helping the handover.
Further, MIES can be classified as a link event service, which deals with the link event starting from an object which generates an event in the lower layer (lower layer below the second layer) and generally ending in MIHF, and MIH event service, which deals with the MIH event transmitted to the upper layer (upper layer over the third layer) registered by the MIHF.
Again, the link event and the MIH event can be classified as two types according to the transmitted region. If the events are generated in the source of event in the local stack and are transmitted upward to local MIHF or the upper layers in the MIHF, these events can be called as ‘local events’. And, if the events are generated in the remote MIHF and are transmitted to remote MIHF, or if the events are transmitted from the remote MIHF to the local MIHF, these events can be called as ‘remote events’.
Next, MICS deals with the command transmitted from the upper layers (over the third layer) to the lower layers (below the second layer) for determining the link states between the upper layers and other MIH users, and for controlling the adjusted operation. And, MICS can be classified as link command service and MIH command service as like the MIES. The link command and MIH command also can be classified as ‘local command’ and ‘remote command’ according to the region transmitted. The local MIH command is generated in the upper layer and is transmitted to the MIHF (for example, MIHF in the upper layer mobility management protocol, or MIHF in the policy engine). The local link command is generated in the MIHF, and is transmitted to the lower layer (for example, media access control layer in the MIHF or physical layer in the MIHF), for controlling the lower layer. The remote MIH command is information generated in the upper layer and is transmitted to the equivalent layer in the remote stack. And, the remote link command is a command generated in the MIHF and transmitted to the lower layer lower than the equivalent layer in the remote stack.
Finally, the MIIS provides similar frame work in the heterogeneous networks for detecting and selecting present various type networks. That is, the MIIS provides detailed information about the network for detecting and selecting the networks, and it should be accessed from any network. And, the MIIS comprises the following information elements.                Link access parameter        Security mechanism        Neighbor Map        Location        Provider and other Access Information        Cost of link        
As an example of the above information service, there is a MIH_Get_Information.request/response primitive provided by the MIH of the MS, and tables 5 and 6 show the format of these primitives.
TABLE 5NameTypeValid RangeDescriptionInfoQueryAn integer value corresponding to one of theN/AThe type of query thatTypefollowing:is specified1: TLV2: RDF_DATA3: RDF_SCHEMA_URL4: RDF_SCHEMAInfoQueryQuery type specific parametersN/AQuery type specificParametersparameters whichindicate the type ofinformation the clientmay be interested in.
TABLE 6NameTypeValid RangeDescriptionInfoQueryAn integer value corresponding to one of theN/AThe type of query thatTypefollowing:is specified1: TLV2: RDF_DATA3: RDF_SCHEMA_URL4: RDF_SCHEMAMIH_REPORTStringN/AReport consisting ofinformation requestedby the MIH User.StatusEnumerateN/ASpecifies whether theinformation wassuccessfully retrievedor not.
FIG. 3 shows a process of adjusting uplink parameters by a MS through an initial ranging before the MS finishes the network registration.
Each step of the FIG. 3 is described in the following.
A MS acquiring downlink synchronization and information for uplink channel through DL-MAP, UCD, DCD (steps (1)-(2)). Then, the MS scans the UL-MAP, confirms the interval for the initial ranging, randomly selects a code, and transmits the selected code to the BS (steps (3)-(5)).
When the BS sets the backoff window for the initial ranging in the UCD message for protecting the collision of the ranging codes, the MS waits a randomly selected amount of time in that backoff window, and then transmits the code again. Here, when the MS does not receive the reply to the ranging code transmitted by the MS from the BS until the T3 timer expires, the MS increases the backoff window size up to the twice, and transmits another code to the BS (steps (6)-(7)).
When the MS receives the ‘continue’ as a ranging state and the code parameter transmitted in the RNG-REQ message, the MS readjusts uplink parameters by the parameters in the RNG-RSP message. By repeating the transmitting code and receiving RNG-RSP message until the ranging state becomes ‘success’, the MS finishes adjusting uplink parameters such as time, power, frequency for the uplink, and the BS allocates bandwidth by the CDMA allocation IE when transmitting UL-MAP for the mobile station to transmit RNG-REQ message (steps (8)-(11)).
Then, the MS transmits RNG-REQ message comprising MAC address and MAC version through the allocated bandwidth (step (12)). And, the BS transmits to the mobile station the RNG-RSP message allocating basic CID and primary CID to the mobile station as an initial ranging CID (step (13)).
On the other hand, a periodic ranging is performed after finishing the network entry process by the MS with the BS, so the periodic ranging has the process same to the process of initial ranging except excluding the steps of (12) and (13).
Total 256 ranging codes are consisted of domains for initial ranging, periodic ranging, bandwidth request ranging and handover ranging, and the MS selects a code in each domain.